Many types of communications are possible over data networks, including electronic mail, web browsing, file downloads, electronic commerce transactions, text chat, and so forth. Networks include private networks, such as local area networks (LANs) or wide area networks (WANs), and public networks, such as the Internet.
With the increased capacity and reliability of packet-based data networks, voice communications (including telephone calls, video conferencing, and so forth) over data networks have become possible. Voice communications over packet-based data networks are unlike voice communications in a conventional public-switched telephone network (PSTN), which provides users with dedicated, end-to-end circuit connections (e.g., one or more time slots of a given channel) for the duration of each call. In a circuit-switched network, speech and control data are routed over the dedicated circuit connection during the call session. In contrast, communications over packet-based data networks, such as IP (Internet Protocol) networks, are performed using packets or datagrams that are sent in bursts from a source to one or more destination nodes. In a packet-based network, a dedicated channel portion is not provided; instead, the communications network is shared, with routing of packets performed based on addresses carried in the packets.
The types of equipment that may be used to perform telephony communications over packet-based data networks include computers that have voice processing components, including a speaker and a microphone. Such computers also include audio coder/decoders (CODECs), implemented in hardware and/or software, that convert between audio and digital data. The telephony application running in such a computer is often referred to as a soft client application, and the computer itself may be referred to as a “soft phone.” One example soft client application is the i2050 product from Nortel Networks Limited.
Another type of device that is capable of telephony communications over a packet-based data network is a network telephone, such as the i2004 telephone set from Nortel. A network telephone may include a network interface and various layers (including network and transport layers) to enable communication of voice data over the data network.
In conventional circuit-switched telephony networks, telephones are coupled to a switch, such as a private exchange system (e.g., a PBX or private branch exchange system, a key telephone system, and so forth). A signaling channel between the switch and the telephone carries control signaling such as an alert message (to indicate an incoming call), commands to update the telephone display, an off-hook indication, an on-hook indication, an on-hold indication, signals carrying button or key presses of the telephone, and so forth. Once a call is established, a voice channel carries voice traffic between the switch and the telephone. The signaling and voice channels are physically associated with the switch port number that the telephone is plugged into. In the switch, a directory number (DN) of the telephone is also associated with the switch port.
With the advent of packet-based network telephony (e.g., IP telephony), telephone sets (e.g., network telephones or soft phones) are no longer connected directly to a switch. Instead, the telephone sets are coupled over a LAN, WAN, or Internet to a system running a Terminal Proxy Server (TPS). In most cases, the TPS resides in the same system as the switch (e.g., a PBX). The TPS acts as a proxy server on behalf of the various telephony clients (which are the telephone sets). The TPS reserves a logical port in the switch for the telephony client, and routes call control signaling messages and other traffic between the telephony client and the switch through this logical port.
A switch such as a PBX or key telephone system offers various services to users, including voice mail, telephony features such as call forwarding and speed dial, computer based applications such as database lookup, and so forth. These services are accessible from the telephone set that is connected to the switch, such as in an office network. However, when a user is not at the office (e.g., at home or traveling), many of these services are generally not available to the user.